The present invention applies to the monitoring of lateral guidance orders being provided for a flight control system of the aircraft and being generated by a calculation stage for lateral guidance orders of an aircraft guidance system. Such a calculation stage comprises, in general, more specifically:                a large feedback loop, determining, from guidance parameters, a turning initiation order corresponding to a roll control order; and        a small feedback loop, determining, from said roll control order, lateral guidance orders of the aircraft, provided for said flight control system.        
Although not exclusively, the present invention more particularly applies to operations with required navigation performance with authorization required, of the Required Navigation Performance with Authorization Required (“RNP AR”) type. These RNP AR operations are based on a surface navigation of the aRea NAVigation (“RNAV”) type and on required navigation performance operations of the Required Navigation Performance (“RNP”) type. These operations have the particular feature of requiring a special authorization for being able to be implemented on an aircraft.
The RNAV type surface navigation allows an aircraft to fly from a waypoint to another waypoint, and no longer from ground stations (of radio-navigation means of the NAVAID type) to other ground stations.
As known, the RNP concept corresponds to a surface navigation, for which (on board the aircraft) monitoring and warning devices are added, allowing to ensure that the aircraft remains in a corridor, referred to as RNP, around a reference trajectory and authorizing taking into consideration curved trajectories. Outside this corridor, potentially relief or other aircrafts could be present. The performance required for a RNP operation type is defined by a RNP value representing half the width (in nautical miles: NM) of the corridor around the reference trajectory, in which the aircraft should remain 95% of the time during the operation. A second corridor (around the reference trajectory) of half a width twice the RNP value is also defined. The probability that the aircraft goes out of this second corridor should be lower than 10-7 per hour of flight.
The concept of RNP AR operations is still even more stringent. The RNP AR procedures are indeed characterized by:                RNP values:                    being lower than or equal to 0.3 NM in approach, and that could go down to 0.1 NM; and            being strictly lower than 1 NM at the start and during a throttling up, and that could also go down to 0.1 NM;                        a final approach segment that could be curved; and        obstacles (mountains, traffic, . . . ) that could be located at twice the RNP value with respect to the reference trajectory, while for usual RNP operations, an additional margin with respect to obstacles is provided.        
The air authorities have defined a target level of safety TLS of 10-7 per operation, whatever the type. In the case of RNP AR operations, as the RNP values can go down to 0.1 NM and the obstacles could be located at twice the RNP value of the reference trajectory, this objective results in a probability that the aircraft goes out of the half-width corridor D=2.RNP that should not exceed 10-7 per procedure.
The equipment embedded on board aircraft (flight management system, inertial unit, devices for updating GPS data and devices for guiding the autopilot), as well as known architecture, do not allow to reach the target level of safety, if mitigation operational means are not provided, including for detecting and managing possible breakdowns. This is why a special authorization is required for this type of operation, so as to ensure that the operational procedures and the pilots' training allow the target level of safety to be reached. Moreover, as the crew should take in charge some breakdowns, the aircrafts are currently not able to guarantee a RNP value of 0.1 NM in a breakdown situation, as the crew are not able to meet the performance requirements in manual piloting.
On current aircrafts, the monitoring of RNP AR operations is implemented by means of two known functions, that is:                a first function monitoring the accuracy and the integrity of the position calculation:        the accuracy of the position is compared to a threshold of the RNP value;        the integrity is compared to a threshold of twice the RNP value; and                    if one of the two parameters, accuracy or integrity, exceeds the allotted threshold, a warning is emitted and the crew should take appropriate actions; and                        a second function allowing the crew to monitor the guidance of the aircraft:                    the lateral and vertical deviations of the aircraft with respect to the reference trajectory are displayed and shown to the crew;                        the crew monitors the deviations compared to the budgets allotted for each deviation. If the crew detects an excessive deviation, the crew should keep the aircraft under control again and take the adequate corrective actions.        
As set forth previously, the current aircraft is not able to guarantee a RNP value of 0.1 NM in a breakdown situation and the crew should be trained specially for flying the RNP AR procedures. The crew should, indeed, be able to detect and process adequately breakdowns being able to compromise the ongoing operation.
The objective for future aircrafts is to be able to fly RNP AR procedures with RNP values up to 0.1 NM, and this without restriction (in a normal situation and in the case of a breakdown) in start, approach and throttling up phases. To this end, the crew should no longer be considered as the main impetus for detecting and processing breakdowns.
As set forth above, an aircraft is generally provided with a guidance system comprising at least one calculation stage for guidance orders, being intended to assist a flight control system of the aircraft. Now, for the aircraft to have the ability to fly particular procedures and including RNP AR procedures, it is necessary to be able to remove from the guidance loop an erroneous source of calculation for guidance orders, so as to counteract any detrimental possible effects on the trajectory of the aircraft.
The present invention aims at providing such a solution allowing detecting an erroneous source of calculation of lateral guidance orders. It relates to a method for automatically monitoring the lateral guidance orders of an aircraft, in particular of a transport airplane, being provided with at least one calculation stage for guidance orders, intended for a flight control system of the aircraft.